The invention described herein relates to methods of forming metallic coatings on polymeric or other nonmetallic substrates. In particular, it relates to methods of forming such coatings by sorption and/or diffusion of metals into coatings or films of polymeric material deposited by conventional techniques on a desired substrate. The United States Government has rights in this invention pursuant to Contract W-7405-ENG-36 between the U.S. Department of Energy and the University of California (41 C.F.R. .sctn.9-9.109-6(i)(5)(ii)(B)).
Metal coated articles enjoy a wide utility. Typically, such articles are obtained by chemical or electrolytic processes wherein a compound of the coating metal is reduced to the metal on a desired substrate under controlled conditions. In general, however, the known methods are not entirely satisfactory. They require either high temperatures (above 150.degree. C. to as high as 1500.degree. C.) or are limited to aqueous plating baths or to the use of "active" metal substrates in accordance with the electromotive series and are otherwise not readily adapted to coating a wide variety of substrates, especially those of various plastics or other organic polymeric materials.
This latter defect is presently taught metal coating processes is particularly disadvantageous in certain specialized utilities as, for example, the fabrication of advanced (laser, electron beam, light ion, heavy ion) inertial confinement fusion (ICF) targets. Such targets are exemplified by the Los Alamos National Laboratory Polaris Prime design in which a glass microballoon containing the deuterium-tritium fuel mixture is encapsulated by four distinct layers or shells. The outermost layer is a relatively thick (75 .mu.m), low atomic number, Z (1 to 9) plastic or polymeric shell that absorbs the laser light and provides the ablation atmosphere. This outer shell is deposited directly on an intermediate pusher layer which is a high-Z-loaded plastic or polymeric shell typically 75 .mu.m thick that both shields the fuel from preheat and provides velocity multiplication when it collides with the high-Z metal, e.g., Au, inner pusher which is deposited directly on the microballoon. Between the intermediate pusher layer and the inner pusher layer is a low-Z cushion layer which, depending on the design, requires a density of 0.01 to 1 g/cm.sup.3 or more and can be a gas, a small-cell plastic foam, or a normal-density plastic. In Polaris Prime this cushion is typically 280 .mu.m thick. The fuel is a layer of DT ice almost 10 .mu.m thick deposited on the inner wall of the 1 .mu.m thick glass microballoon.
It is desirable to modulate the pressure profile during the implosion. This can be done by varying the effective atomic number Z as a function of radius in the absorber/ablator layer. To produce the necessary variation, it is necessary to have techniques for forming this layer by depositing high-Z-loaded plastics or other polymeric materials having radial gradients both in Z and in density.
Finally, in order to avoid the development of hydrodynamic instabilities during the implosion, high-quality surface finishes having deviations less than 0.1% of the coating thicknesses and coating thicknesses uniform to at least 1% are required for almost all of the various plastic or polymeric coatings or films contemplated for use in an ICF target.
Although various techniques have been used in the manufacture of ICF targets, the most successful have been the low-pressure-plasma (LPP) coating process, also known as glow-discharge polymerization, plasma polymerization, etc., and the vapor-phase-pyrolysis (VPP) coating process. There is general agreement that the LPP process is extremely complex and critically dependent on the experimental conditions used. Nonetheless, the LPP process has the advantage over the VPP process in that it can coat free-standing microsphere substrates with high coating thickness uniformity (.about.1%) and very smooth surface (a few 10-nm peak-to-valley irregularities). The VPP process, on the other hand, can produce very thick coatings (40 .mu.m and higher) in a reasonably short time with no apparent thickness limitation, but is limited to stalk-mounted substrates for coating thicknesses above about 5 .mu.m. Neither process, however, is entirely conducive to producing graded polymeric coatings or films having an appropriate loading of a desired metal or metals or to producing metal coatings on polymeric substrates.
Accordingly, it is the object and purpose of the present invention to provide a method of forming metal coatings on non-metallic, polymeric substrates.
A further object of the invention is to provide a method of preparing metal coatings on heat sensitive substrates without damaging such substrates.
A still further object of the invention is to provide a method of obtaining mixed metal or alloy concentrations within or on polymeric or plastic coatings or films.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.